In the wishbone type suspension system, a pair of planar swing link arms are disposed in a vertically spaced manner, and a suspension coil spring and a tubular damper are mounted between the lower swing arm and the vehicle body as a shock absorber. Therefore, the wishbone suspension system is known to have the problems that it is complex in structure, heavy, expensive and poor in space efficiency as compared with the strut type and other suspension systems. In particular, when it is used on a front wheel of front drive vehicle, it is necessary to place the damper above the drive shaft or offset therefrom along the fore-and-aft direction so as not to interfere with the drive shaft.
Therefore, Japanese utility model laid open publication No. 56-122842 discloses a double wishbone suspension system which makes use of a torsion bar and a rotary damper instead of a coil spring and a tubular damper. However, since the torsion bar is directly mounted on the rotary damper, it is difficult to achieve a necessary sealing arrangement in the damper against the twisting deformation of the torsion bar. Furthermore, since the link moving vertically with the vertical movement of the wheel and the rotary damper are connected to each other via the torsion bar, the angular displacement of the rotary damper becomes smaller than the swing angle of the link arm. Therefore, the damper must be increased in size in order to obtain a sufficient damping force, and must extend under the torsion bar to such an extent that the height of the vehicle bottom from the road surface becomes unacceptably small.
As an alternate type of suspension system, there is the strut type suspension system which includes a strut unit integrally formed with a steering knuckle generally consisting of a strut incorporating a tubular damper, and a suspension coil spring surrounding the strut. Since the strut unit is subjected to bending forces arising from brake torque and drive torque as well as those arising from lateral forces during cornering manoeuvers, and such bending forces give rise to frictional forces, it is necessary to increase the diameters of the strut and the piston rod of the damper in order to ensure a sufficient rigidity and achieve a desired sealing performance inside the strut.
Therefore, a conventional strut unit is disposed so as to extend inwardly of the vehicle body in order to avoid interference between the damper and coil spring and the wheel. This not only restricts the space in the engine room or the passenger compartment but also prevents reduction in cost due to the complexity of the structure and the need for a large number of component parts. Further, since the strut unit must be relatively heavy, the overall weight of the suspension system is necessarily large, and its center of gravity tends to be high. Moreover, in order to secure a large effective stroke of the damper, the length of the strut unit is increased, and this imposes a severe restriction on the freedom of vehicle body design, particularly because of the increased height of the bonnet line above the front wheel.
In view of this fact, the applicant of the present application actually put into use a strut type suspension system using a torsion bar instead of a coil spring, as disclosed in Japanese patent laid open publication No. 57-191112. There are other previously proposed suspension systems using rotary dampers instead of tubular dampers, as disclosed in Japanese utility model laid open publication No. 56-122842 and Japanese patent publication No. 58-55923.